Re: [PATCH v3 10/10] selftests/io_uring: add bpf io_uring selftests

[Ming Lei <ming.lei@redhat.com>]

On Sat, Nov 22, 2025 at 12:12 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 11/20/25 01:41, Ming Lei wrote:
> > On Wed, Nov 19, 2025 at 07:00:41PM +0000, Pavel Begunkov wrote:
> >> On 11/14/25 13:08, Ming Lei wrote:
> >>> On Thu, Nov 13, 2025 at 11:59:47AM +0000, Pavel Begunkov wrote:
> >> ...
> >>>> +  bpf_printk("queue nop request, data %lu\n", (unsigned long)reqs_to_run);
> >>>> +  sqe = &sqes[sq_hdr->tail & (SQ_ENTRIES - 1)];
> >>>> +  sqe->user_data = reqs_to_run;
> >>>> +  sq_hdr->tail++;
> >>>
> >>> Looks this way turns io_uring_enter() into pthread-unsafe, does it need to
> >>> be documented?
> >>
> >> Assuming you mean parallel io_uring_enter() calls modifying the SQ,
> >> it's not different from how it currently is. If you're sharing an
> >> io_uring, threads need to sync the use of SQ/CQ.
> >
> > Please see the example:
> >
> > thread_fn(struct io_uring *ring)
> > {
> >       while (true) {
> >               pthread_mutex_lock(sqe_mutex);
> >               sqe = io_uring_get_sqe(ring);
> >               io_uring_prep_op(sqe);
> >               pthread_mutex_unlock(sqe_mutex);
> >
> >               io_uring_enter(ring);
> >
> >               pthread_mutex_lock(cqe_mutex);
> >               io_uring_wait_cqe(ring, &cqe);
> >               io_uring_cqe_seen(ring, cqe);
> >               pthread_mutex_unlock(cqe_mutex);
> >       }
> > }
> >
> > `thread_fn` is supposed to work concurrently from >1 pthreads:
> >
> > 1) io_uring_enter() is claimed as pthread safe
> >
> > 2) because of userspace lock protection, there is single code path for
> > producing sqe for SQ at same time, and single code path for consuming sqe
> > from io_uring_enter().
> >
> > With bpf controlled io_uring patches, sqe can be produced from io_uring_enter(),
> > and cqe can be consumed in io_uring_enter() too, there will be race between
> > bpf prog(producing sqe, or consuming cqe) and userspace lock-protected
> > code block.
>
> BPF is attached by the same process/user that creates io_uring. The
> guarantees are same as before, the user code (which includes BPF)
> should protect from concurrent mutations.
>
> In this example, just extend the first critical section to
> io_uring_enter(). Concurrent io_uring_enter() will be serialised
> by a mutex anyway. But let me note, that sharing rings is not
> a great pattern in either case.

If io_uring_enter() needs to be serialised, it becomes pthread-unsafe,
that is why I mentioned this should be documented, because it is one
very big difference introduced in bpf controlled ring.

Thanks,